The invention relates to inhibition of inorganic deposits (scale for example) particularly in petroleum reservoirs, more particularly under HP/HT (high-pressure high-temperature) conditions.
HP/HT reservoirs are defined as those with generally high salinity, a temperature of at least 150° C., and pressures that may exceed 70 bars, for example offshore reservoirs.
Numerous operations in the petroleum industry may give rise to inorganic deposits, particularly producing operations, producing-well stimulation, and transportation. Most of the inorganic deposits in the petroleum sector are linked either to direct precipitation that occurs naturally in reservoir rock or to supersaturation of the producing water that occurs when two incompatible liquids are found downhole.
The deposits can consist of calcium carbonate, barium sulfate, calcium sulfate, strontium sulfate, iron carbonate, iron oxides, iron sulfates, and magnesium salt deposits. Silica deposits may also form, particularly with SAGD (steam assisted gravity drainage) techniques.
Specific problems are linked to HP/HT conditions. Deposits of varied compositions may occur in HP/HT wells, particularly mineral deposits such as barium sulfate, strontium sulfate, and calcium sulfate. The problem of carbonate deposits (particularly calcium) is highly specific due to the drastic reduction in the solubility of calcium carbonate when the temperature rises. The main effect is due to CO2 degassing when the pressure drops during the producing process and to the high salinity (high calcium concentration) in the formation water present in HP/HT reservoirs.
Highly specific deposits may also be noted, particularly in cases where the wells produce gas from the reservoir. These are halite deposits due to very high salinity of the water associated with gas production and water evaporation near the well. These deposits are generally treated by flushing the well bottom with water from the aquifer, or low-salinity water, or even with desulfated water.
Moreover, the production lines in HP/HT reservoirs are subject to wide temperature variations, particularly when offshore. The downhole temperature may be high (150° C.) but at the sea bed the temperature may be about 4° C., meaning significant temperature gradients over distances as far as several thousand meters. In this case, the location of the inorganic deposits results from both thermodynamic and kinetic effects whose amplitudes are temperature-dependent. For example, for barium sulfate, the solubility increases with temperature, making deposition highly probable at low temperatures, but the precipitation rate decreases with temperature, limiting the extent of the precipitation.
Well flushing by injecting desulfated water (20-100 mg/L) does not solve every problem, however. When the sulfate concentration is low in the injected seawater, barium sulfate deposits may occur, but in smaller quantities. For these reasons, an inhibitor of inorganic deposit formation is generally added to the flushing water to prevent any risk of precipitates (generally sulfates) forming when the aquifer water is mixed with the highly saline brine.
Deposition inhibitors prevent precipitation by two main mechanisms:                inhibition of nucleation: they act as dispersants to prevent formation of nuclei;        growth inhibition: they prevent the growth of crystals by adsorption on crystal sites. They can act as dispersants to limit aggregation.        
The scale inhibiting molecules may have sizes adapted to the crystallographic size of the designated salt. It is generally considered that phosphonates are crystal growth inhibitors, while polymers act as dispersants. Most commercial products are mixtures of growth and nucleation inhibitors. Scale inhibitors are effective at very low concentrations. There is a threshold effectiveness concentration called the Minimum Inhibitory Concentration (MIC).
Inhibitor Retention
The scale inhibitors are retained downhole by two main mechanisms: retention by adsorption and/or retention by precipitation. The retention mechanisms depend on the chemical nature of the substance, the concentration, and reservoir-dependent parameters such as the composition of the reservoir water, the mineralogy of the rock, pH, and temperature. The adsorption process involves electrostatic adsorption mechanisms or Van der Waals bonding mechanisms. The pH and divalent ion (calcium) concentration are the predominant factors that enhance adsorption. In certain cases, the divalent cations are also responsible for surface or mass precipitation, which increases retention.